1. Field of the Invention
The present invention relates to an asymmetrical flat antenna, a method of manufacturing the asymmetrical flat antenna, and a signal-processing unit using this antenna.
2. Description of Related Art
An antenna has been often designed and developed so that it is suitably used in an electronic device having portability, a communication device that is used in a home where it is difficult to wire a communication cable therein or is used for a long distance in a precinct or a building, and the like. That is, the development and design of the antenna have been often carried out aiming at how efficiently to emit radio wave to a long distance and for its basic design, emission of radio wave to a long distance has been considered. As a design having broadband performance in such antenna development, an antenna which indicates constant impedance throughout wide frequencies can turn to an antenna having broadband performance so that self-similar antenna, self-complementary antenna and the like have been developed.
Generally, the antenna is preferred to pick up radio wave from a certain direction with an excellent sensitivity based on its directivity while avoiding picking of radio waves from other directions. However, radio wave emitted from a transmission antenna or radio wave emitted from other electronic component induces multi-reflection of radio wave under an environment substantially surrounded by metals, thereby producing multipath.
For example, under an environment in which the antenna and a signal-processing board including the antenna or large scale integrated (LSI) circuit device are substantially surrounded by metals, signal deterioration due to multipath may often occur. If a receiving antenna receives radio wave under such the multipath, the receiving antenna may mix radio wave received directly with it, thereby resulting in deterioration of signal quality because the receiving antenna fails to recognize a preferred one of their waveforms.
FIG. 1 shows a configuration of a rectangular patch antenna 10 as a related art example. The rectangular patch antenna 10 shown in FIG. 1 includes multilayered board 8 overlaid with a grounding layer, not shown, (hereinafter referred to GND layer) and an insulator layer 7, and a rectangular antenna pattern 6 thereon. The rectangular patch antenna 10 indicates narrow band performance because it has the GND layer on the bottom layer of the antenna pattern 6 via the insulator layer 7. The antenna pattern 6 has a length of λ/2 at a longitudinal direction thereof where the wavelength of frequency to be used is λ. If the antenna pattern 6 is divided into two parts each having a length of λ/4 on both of the right and left sides with respect to the center of power supply pattern 1, a shape of the part having a length of λ/4 on the left side and that of the part having a length of λ/4 on the right side are identical to each other.
Japanese Patent Application Publication No. 2005-192183 has disclosed an antenna for ultra-wide band communication in association with this kind of the rectangular patch antenna 10. Such the antenna is constructed to have a patch and a ground area on its substrate. The patch is formed smaller than the substrate. When supplied with current via a feeder, the patch is excited to emit any energy. The ground area is formed by removing area on the surface of the substrate other than the patch to have broadband performance.
Japanese Patent Application Publication No. 2004-220264 has disclosed an electronic device equipped with the rectangular patch antenna 10. The electronic device has a main body case and this main body case contains a mother board having a device composition unit. It also has a daughter board constituting other device composition units and each of the device composition units is provided with a broadband communication chip. The interior of the main body case is covered with radio wave absorption body to absorb electromagnetic wave which may induce communication noise between the device composition units.